by Tim Kiska

Rick Santorum held a 39 percent- 24 percent lead over competitor Mitt Romney a little more than a week ago. But that lead has dropped to 37-33, indicating that the momentum is on Romney’s side.

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The Public Policy Polling Survey shows a 10 percent jump in Romney’s favorability rating in just one week. 

“What we’re seeing in Michigan is a very different story from Florida where Romney surged by effectively destroying his opponent’s image,” writes Public Policy’s Tom Jensen. “Here Romney’s gains have more to do with building himself up.”

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Santorum is running strong among Protestants, Tea Party fans and union members—who are undoubtedly unhappy about Romney’s “Let Detroit Go Bankrupt” stance. (For the record, Santorum position differs little from Romney’s.)

New York Times survey expert Nate Silver says the Romney momentum factor may be overstated. But he adds that the natural Romney constituency may be returning to the fold.

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Jensen says Michigan’s February 28 primary is still an extremely volatile race. And Silver suggests that Wednesday’s debate in Arizona could prove decisive.